1. Field of the Invention
This invention relates generally to medication infusion pumps of the type for implantation directly into the body of a patient and for programmed operation to deliver medication to the patient, and more particularly to an improved and simplified fluid pressure reservoir for maintaining a supply of a selected medication under controlled pressure conditions in an improved implantable infusion pump.
Medication infusion pumps are generally known in the art for use in delivering a selected medication to a patient in a scheduled or preprogrammed manner. In recent years, such infusion pumps have been developed in compact form adapted for direct implantation into the body of a patient, to deliver a specific medication such as insulin to the patient in discrete doses over an extended time period. An implanted infusion pump of this general type includes an internal medication chamber for receiving and storing a supply of the selected medication in liquid form, with the medication being subjected to a predetermined storage pressure to ensure accurate and repeatable delivery conditions through the use of a miniature pump and associated programmed control means. In many cases, the storage pressure is maintained at less than ambient body pressure to prevent undesired leakage of the medication from the medication chamber into the body of the patient, and to thereby positively prevent accidental overdose during certain failure modes. For one illustrative example of an implanted medication infusion pump of this general type, see U.S. Pat. No. 4,573,994, to Fischell.
In the past, the medication within the pump medication chamber has been subjected to the desired storage pressure by forming at least a portion of the medication chamber as a movable wall shared with a pressure reservoir charged with a pressure fluid. More particularly, the pressure fluid has comprised a selected fluid in a liquid-vapor state, such as a selected fluorocarbon, wherein the pressure fluid undergoes liquid-vapor change of state at normal body temperature to appropriately expand or contract the pressure reservoir in a manner acting through the movable wall to maintain the medication chamber under substantially constant pressure conditions.
As the medication chamber is filled, the pressure fluid undergoes significant state change to the liquid phase to reduce the volumetric size of the pressure reservoir. Conversely, as the medication is delivered in doses to the patient, the pressure fluid progressively undergoes state change to the vapor phase to maintain the medication under substantially constant pressure. Freon 113 has been used to maintain the medication at a slight negative or subambient pressure in response to normal patient body temperature and altitudinal variations up to about 8,500 feet above sea level.
While such implantable infusion pumps constitute a major step forward in reliable and convenient administration of certain medications, some design aspects of such pumps have contributed to a relatively complex and costly pump construction. For example, the movable wall or barrier separating the pressure reservoir from the medication chamber has been constructed from a metal material to permit leak-free attachment to the pump housing which has also been constructed from a biocompatible metal material, particularly such as titanium or titanium alloy.
Efforts to utilize structurally simple metal foil diaphragms have proven unsatisfactory due to fatigue failures encountered as such diaphragms are displaced during filling and subsequent normal pump operation. Instead, current implantable pumps have utilized metal bellows devices formed from annular metal rings which are interconnected by a series of thin circular welds requiring an extremely high degree of precision and costly manufacturing equipment. Moreover, in use, such metal bellows structures have encountered occasional and highly undesirable fatigue failures. Further, the bellows devices tend to occupy a substantial portion of the internal pump housing volume, resulting in significant limitations in medication capacity or otherwise resulting in a pump housing of relatively large size.
There exists, therefore, a significant need for further improvements in and to implantable medication infusion pumps, particularly with respect to improvements in the design and operational reliability of the movable wall or barrier separating the medication from a liquid-vapor pressure fluid within a pressure reservoir. The present invention fulfills these needs and provides further related advantages.